Scientists have discovered what causes cataclysm-inducing supervolcanoes to erupt, and the answer offers little reassurance. Their eruptions are caused by magma buoyancy, which makes them less predictable and more frequent than previously thought.

A team of geologists from the Swiss Federal Institute of
Technology in Zurich (ETH) modeled a supervolcano – such as
Yellowstone in Wyoming – using synthetic magma heated up with a
high-energy X-ray to see what could create a powerful discharge.
A separate international team, led by Luca Caricchi of the
University of Geneva, conducted more than 1.2 million computer
simulations of eruptions.

Both groups have arrived at similar conclusions, with twostudies simultaneously published in Nature
Geoscience magazine.

"We knew the clock was ticking but we didn't know how fast:
what would it take to trigger a super-eruption?” said Wim
Malfait, the lead author of the ETH study.

"Now we know you don't need any extra factor - a supervolcano
can erupt due to its enormous size alone.”

It was previously thought that supervolcanoes – which spew out
hundreds more times of lava and ash than ordinary ruptures –
could be triggered by earthquakes or other outside tectonic
phenomena.

It was also clear that these volcanoes do not operate like
ordinary eruptions, which rely on magma filling their chambers,
and spurting through an opening, once the pressure gets to a
certain point, since the chambers of supervolcanoes are too large
to be over pressurized to the same degree.

Now, the studies have identified the unique supervolcano
mechanism that makes their discharge more like powerful
explosions than normal eruptions.

The molten magma in the mostly underground supervolcano is
lighter than the surrounding rocks, and the difference in
pressure, creates a 'buoyancy effect', meaning the super-hot
terrestrial soup is always attempting to burst out.

“The difference in density between the molten magma in the
caldera and the surrounding rock is big enough to drive the magma
from the chamber to the surface,” said Jean-Philippe
Perrillat of the National Centre for Scientific Research in
Grenoble, where the experiments were conducted.

“The effect is like the extra buoyancy of a football when it
is filled with air underwater, which forces it to the surface
because of the denser water around it. If the volume of magma is
big enough, it should come to the surface and explode like a
champagne bottle being uncorked.”

The researchers believe that the pressure force of the molten
magma pools can be strong enough to crack 10 km thick layers of
rock, before spewing out a maximum of between 3,500 and 7,000
cubic kilometers of lava. In comparison, the notorious Krakatoa
explosion in 1883 likely ejected less than 30 cubic kilometers of
debris into the atmosphere.

The effects on Earth are likely to be fundamental, with previous
studies suggesting that such a supervolcano
could decrease the temperature on Earth by 10 C for a decade, as
the ash would prevent sunlight from reaching the ground.

The last supervolcano eruption in Lake Toba took place more than
70,000 years ago. According to one highly-contested theory it may
have wiped out more than half of the planet’s population; in any
case the effect on the world would be dramatic.

"This is something that, as a species, we will eventually
have to deal with. It will happen in future," said Dr
Malfait.

"You could compare it to an asteroid impact - the risk at any
given time is small, but when it happens the consequences will be
catastrophic."

A volcano has to eject more than 1,000 cubic km of debris in a
single eruption to be counted as a supervolcano, and there are
less than ten potential sites with sufficiently large magma
chambers around the world, though there may be others lurking
underneath the ocean surface. These formations, which are more
often flat with no outlet, are expected to erupt once every
50,000 years, though there is no regularity to the frequency of
eruptions.

The computer modelers believe that the buoyancy mechanism means
that such eruptions occur more frequently than previously
thought, though the exact extent is hard to estimate without
studying magma flows at each potential location.

Nonetheless, the ETH scientists say that there could be
detectable pressure changes, and perhaps even spectacular rises
of ground level sometime before the eventual explosion. But it is
not clear how long after such changes an eruption would take
place, or whether advance knowledge would actually help to
mitigate its impact.